Matthew McConaughey has had the same perm since he was 15

Matthew McConaughey's hair has stayed curly ever since he had a perm as a teenager.

The Interstellar actor could be recognised anywhere with his signature leather jacket, trademark swagger and mop of wavy brown hair. But it turns out Matthew's curls might be the result of an intense perm he had done in the mid-1980s, at the height of the fad for big hair styles.

"I graduated high school in 1988. I remember in '85 I got the perm," he told Entertainment Tonight. "I think my hair was straight before I got the perm, and I don't think it's ever gone back to being straight since I got the perm."

The Oscar-winning star is currently promoting new film Gold, which is based on the true story of the 1993 Bre-X mining scandal, when a massive gold deposit was supposedly discovered in the jungles of Indonesia. Matthew plays unlucky businessman Kenny Wells in the film, who teams up with geologist Michael Acosta (Edgar Ramirez) with a plan to get their hands on the precious metal.

The 47-year-old had to pack on the pounds for the part, but he admits that putting on weight was much easier than shedding, as he did when he portrayed Ron Woodroof, a real-life AIDS patient, in 2013 movie Dallas Buyers Club.

"I said, 'Six months, you say yes to whatever you want to eat, whatever you want to drink, 24/7. If you think twice about it, McConaughey, you have to have double.' So it happened kind of easily," he said of his weight gain plan.

He added that the eating regime made him very popular with his three children, Levi, eight, Vida, seven and four-year-old Livingston, whom he shares with his model wife Camila Alves.

"The whole family loosened up, my nickname was 'Captain Fun' around the house because every night was pizza night if the kids wanted it, milkshakes for breakfast was just fine," he smiled.